Generally, it is contemplated that an internal combustion engine where NOx (nitrogen oxides) is discharged due to combustion in cylinders is provided with an occlusion-reduction type NOx catalyst for occluding NOx in a lean state and reducing/releasing NOx in a rich state to purify NOx in exhaust gas.
Although the NOx catalyst occludes NOx when an air/fuel ratio of atmosphere of exhaust gas is lean, the NOx occlusion capacity of the NOx catalyst will become low with the NOx occlusion amount approaching the limit of the occlusion capacity.
Therefore, in order to reduce and remove NOx having been occluded by the NOx catalyst and restore the NOx purification capacity of the NOx catalyst, a process (rich purge control) for reducing and removing NOx having been occluded is performed. In this case, the air/fuel ratio of atmosphere of exhaust gas is set rich and a reducing agent such as HC or CO is supplied to the NOx catalyst when the NOx occlusion amount of the NOx catalyst reaches a threshold value.
Moreover, when the internal combustion engine has been used for the long term, sulfur in fuel is adsorbed to the NOx catalyst so that a sulfur poisoning occurs. Therefore, the purification capacity of the NOx catalyst will become significantly low. Thus, with reference to JP-2000-34946A, a technology is proposed to evaluate the deterioration of the purification capacity (catalyst deterioration evaluation) of the NOx catalyst in accordance with the performing of the rich purge control. Specifically, an oxygen concentration sensor is arranged at a downstream side of the NOx catalyst, to perform the catalyst deterioration evaluation based on the detection result of the oxygen concentration sensor when the rich purge control is performed.
That is, in the rich purge control, the air/fuel ratio of the downstream side of NOx catalyst is switched into rich when the reduction of NOx occluded by the NOx catalyst is finished. Therefore, the finish of the reduction of NOx is determined by detecting the air/fuel ratio via the oxygen concentration sensor. In this case, the switching timing of the air/fuel ratio via the oxygen concentration sensor becomes early when the NOx occlusion capacity becomes low, that is, when the NOx amount which can be occluded by the NOx catalyst decreases. Therefore, the deterioration degree of the purification capacity of the NOx catalyst can be estimated based on the time having elapsed until the air/fuel ratio is switched.
There are two methods for the rich purge control. The first method (combustion purge control) is setting the air/fuel ratio of atmosphere of exhaust gas to be rich to supply fuel which has not been combusted to the NOx catalyst as the reducing agent, by increasing the injection amount of fuel into the cylinders of the internal combustion engine to set the air/fuel ratio to be rich. The second method (exhaust addition purge control) is supplying fuel which has not been combusted to the NOx catalyst as the reducing agent, by adding fuel from a fuel supply valve (arranged at exhaust pipe) into the exhaust pipe.
The operation field of the engine where the combustion purge control can be used is limited to the field where the engine has a low RMP and a low load, because noise, vibration and the like in the case of switching a normal state will be caused and excessive smoke will be discharged in the combustion purge control. On the other hand, the exhaust addition purge control is useful, for example, in the case where the increase of the fuel injection amount to the internal combustion engine is not suitable. In this case, the combustion purge control and the exhaust addition purge control are selectively performed, in response to the operation state of the engine when the NOx occlusion amount (which is condition for starting rich purge control) reaches the threshold value.
Thus, in the case of the exhaust addition purge control, there may be an error in the result of the catalyst deterioration evaluation which is performed in accordance with the rich purge control, because only HC as the reducing agent becomes excessively dense at the NOx catalyst when fuel is directly added into the exhaust pipe through the fuel supply valve. Therefore, the catalyst deterioration evaluation is performed by only using the information of the combustion purge control.
However, in this case, the threshold value of the NOx occlusion amount which is the condition for starting the rich purge control is set across-the-board irrespectively of the combustion purge control/exhaust addition purge control. That is, the same threshold value is provided for the combustion purge control and the exhaust addition purge control. Therefore, it is difficult to sufficiently ensure the occasion where the combustion purge control is performed. Therefore, it is also difficult to sufficiently ensure the occasion where the catalyst deterioration evaluation is performed.